Vehicle drive shafts are employed to connect the universal joints at the front and rear of the vehicle. In the past, the drive shaft has commonly been formed of a steel tube having yokes connected at its ends. More recently, to reduce the overall weight of the vehicle, drive shafts have been made of fiber reinforced resin with the fibrous material consisting primarily of glass and graphite fibers. Graphite fibers have the advantage of providing stiffness in the longitudinal direction but are more expensive than glass.
In the design of a drive shaft it is necessary that the longitudinal bending mode be above a certain frequency. If it is not above this frequency, vibrations can occur and result in possible failure of the drive shaft.
As a further requirement, the drive shaft must be capable of carrying the torque of the engine without shear or torsional failure. If the drive shaft is not capable of carrying the engine torque, torsional buckling can occur which results in a deformation of the cross-section of the drive shaft and then a collapse and failure of the shaft at a given maximum torque.
Torsional buckling is more prevalent on longer length drive shafts which can, in some instances, be in the neighborhood of 90 to 100 inches. Torsional buckling can be prevented by incorporating additional quantities of graphite fibers to increase the longitudinal stiffness, but this substantially increases the overall cost of the drive shaft. A second manner of preventing torsional buckling is to construct the drive shaft in shorter lengths which are supported by intermediate bearings. Again the use of bearings contributes to the overall cost of the drive shaft.
Thus, there has been a need for an inexpensive, fiber reinforced resin drive shaft having increased resistance to torsional buckling.